1. Field of the Invention
This invention relates to the production of ethylene from methane and to catalysts used therefor.
2. Description of Related Art
Methane is a naturally occurring substance and is the main constituent in natural gas. It is presently an abundant resource available at a modest cost.
Ethylene is a gas which is one of the basic building blocks of the petrochemical industry for making such products as polyethylene. Accordingly, considerable research has been directed toward converting low cost methane to ethylene. Conventionally the methane is reacted with oxygen to form ethylene and water. The objects of the research are a high selectivity to ethylene, and perhaps other hydrocarbons, a high conversion per pass of CH.sub.4, and a high throughput overall.
Much of the research has focused on finding the best catalyst for the reaction. Many oxides have been tested. All are most active at about 1000 K. Some of the best reported combinations are Sm.sub.2 O.sub.3, CeO.sub.2 /1% Ba, BeO/1% Li, La.sub.2 O.sub.3 /1% Sr, Al.sub.2 O.sub.3 /20% Pb, and LaAlO.sub.2.
Two of the best catalysts found have been SrCe.sub.0.9 Yb.sub.0.1 O.sub.2.95 and MnO.sub.2 /20% LiCl. However, for both of these catalysts, the space time yield (.mu. moles/sec g) is very low, suggesting that the flow rate must be kept very low. These catalysts are discussed in an article by G. J. Hutchings, M. S. Scurrell and J. R. Woodhouse, Chem. Soc. Rev. 18, 251 (1989).
According to Y. A. Amenomiya, V. I. Birss, M. Goledzinowski, J. Galuazka and A. R. Sanger, in "Conversion of Methane by Oxidative Coupling", report to CANMET, Energy, Mines and Resources, Ottawa, Canada, the various promising catalysts are rated in order as Li/Sm.sub.2 O.sub.3 &gt;Na/CaO&gt;K/CaO&gt;LaAl.sub.2 O.sub.3 &gt;Sm.sub.2 O.sub.3 &gt;Li/CaO&gt;PbO&gt;Bi.sub.2 O.sub.3 &gt;Ho.sub.2 O.sub.3 &gt;Gd.sub.2 O.sub.3 &gt;Li/MgO&gt;Li/CaO.about.Yb.sub.2 O.sub.3 &gt;Y.sub.2 O.sub.3 Na/MgO.about.CaO&gt;MgO. Additives to the catalysts include Ba, Li, Sr, Pb, K, Mg, Ca, Na, and Sb.
Many other catalysts have been tested for methane oxidative coupling including lithium deposited on .alpha.Al.sub.2 O.sub.3. However, the conversion rate was found to be low. The consensus in the literature has been that an acid catalyst must be poor, and because Al.sub.2 O.sub.3 is highly acid, there has been little interest in this possibility.
In short, while many catalysts have been explored for converting methane to ethylene, all of those tested have fallen short of the goals of high selectivity to ethylene, a high conversion per pass of CH.sub.4, and a high throughput overall.